FIG. 1 schematically represents a conventional application of a voltage controlled oscillator. A phase locked loop (PLL) serves the purpose of demodulating signals or of synthesizing frequencies. The voltage controlled oscillator VCO produces a frequency Fo determined by a control voltage Vc. Voltage Vc is generated by a phase-frequency comparator PFD associated with a charge pump and a low-pass filter LPASS. The phase-frequency comparator compares the frequency Fo, divided beforehand by N by a frequency divider DIV-N, with a reference frequency Fref, whereas the charge pump supplies a current proportional to the phase difference between a set point and the divided output. This current is integrated via a filter supplying a mean frequency correction voltage value to reach the setpoint. The loop then acts on oscillator VCO so that the frequency Fo tends to N·Fref.
FIG. 2 schematically represents a conventional voltage controlled oscillator called cross-coupled transistor pair oscillator. An “energy tank” comprises an inductor L and a variable capacitor or “varactor” C in parallel. An active impedance Za is connected to the terminals of the energy tank. The oscillator output signal, a differential signal, is taken at the terminals of energy tank LC.
In this example, inductor L comprises a mid-point connected to a high supply line Vcc. The capacitance of varactor C is tunable around a nominal value by a control voltage Vc. A stray resistor Rp is shown connected in parallel with tank LC.
The role of active impedance Za is to produce a negative resistive component at the terminals of energy tank LC tending to compensate the effect of stray resistor Rp so as to maintain oscillation at the resonance frequency of tank LC.
Active impedance Za comprises a pair of cross-connected MOS transistors M1 and M2 whose drains form the respective terminals of the active impedance. The gates of transistors M1 and M2 are respectively connected to the drains of transistors M2 and M1. The sources of transistors M1 and M2 are connected to a reference voltage line GND by a common current source Ib.
One difficulty encountered in the use of this type of oscillator is matching its frequency tuning range to the required applications. For example, in a PLL, the frequency tuning range corresponds to the PLL capture range, which should be as broad as possible. It is on the other hand also desirable for the oscillator tuning slope (noted KVCO), expressed in Hz/V, not to be too steep in order to reduce the sensitivity of the PLL, in particular to noise.
FIG. 3 is a graph of the frequency Fo versus the control voltage Vc of a voltage controlled oscillator, illustrating a conventional approach used to attempt to satisfy these contradictory requirements. The desired bandwidth BW is represented by the grey zone. The oscillator frequency should scan this whole band, with a tolerance margin at each end to compensate for dispersions of the characteristics of the components (due to imperfections of the manufacturing processes and to thermal effects).
The characteristic of an oscillator with a “large swing” varactor, allowing the oscillator to scan the full required range, has been represented by a bold line. In some applications, the oscillator tuning slope may prove to be too steep making it too sensitive to noise.
A series of characteristics corresponding to an oscillator with a moderate swing varactor has been represented by dashed lines. A series of switched capacitors are piggy-backed on the varactor, the capacitors being progressively connected in parallel on the varactor by a control circuit as the oscillator frequency is reduced. Several adjacent tuning ranges or bands are thus created, each with a moderate slope, enhancing the noise immunity of the oscillator.
This may become difficult to implement when the oscillator is to be used at frequencies of several tens of gigahertz, as used in millimetric wave communications (for example 60 GHz for the HDMI wireless standards or 77 GHz for automobile radars). Indeed, each switched capacitor introduces a stray capacitance tending to reduce the oscillation frequency. Furthermore, the sizes of the required switched capacitors are so small that it is difficult to control their values with sufficient accuracy, in particular to ensure that the tuning ranges overlap and cover the full desired spectrum.